1) Field of the Invention
The present invention relates to an optical add/drop multiplexer, and more particularly to an optical add/drop multiplexer in which a wavelength cross-connect function in a wavelength multiplexed optical transmission system and an optical add/drop function can be expanded.
2) Description of the Related Art
In recent years, with increasing traffic volume, there are demands for a large-capacity network. To meet the demands, an optical network using wavelength division multiplexing (WDM) is applied to a conventional basic network. In the optical network, the needs for a wavelength cross-connect function and an optical add/drop multiplexer (OADM) are increasing. With the wavelength cross-connection function, a destination to which an input light is output is changed for each wavelength of WDM light. Such a technology is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. H8-195972. With the OADM, a signal light having an arbitrary wavelength is added to an arbitrary path, and then, dropped. Thus, the signal light is received. The OADM includes a wavelength selective switch (WSS). There are several types of the WSS such as one having a diffraction grating and a matrix switch using a micro electro mechanical system (MEMS) mirror using a MEMS technology, and one having a thin film filter and a matrix switch using the MEMS mirror.
From the viewpoint of a size and a cost of a device having the functions in the wavelength cross-connect function and of the OADM, it is preferable to make such functions expandable as required while the device is configured as small as possible upon its introduction, not just making the functions advanced. When the device is replaced with another one, optical fibers connected to the device have to be reconnected to the one replaced. However, because the number of optical fibers is as many as thousands, it takes a lot of time for the reconnection. Moreover, to carry out the reconnection, the signals being transmitted have to be disconnected. Therefore, it is desirable to realize a configuration (in-service upgrade) such that the functions can be expanded without disconnecting the signals being transmitted.
However, in the conventional configuration, a device is prepared by the number estimated, when a device is to be introduced, corresponding to the number of wavelengths and the number of switching routes to be demanded in the future. As a result, a size of the device required at the time of initial introduction becomes large, and introduction cost of the device at the time of initial introduction is increased.
FIG. 59 is a schematic of a transmission path and a wavelength cross-connect device in a network. Two rings of transmission paths A and B are connected to a wavelength cross-connect device 1300 that forms an optical add/drop multiplexer. The transmission path A includes two optical fibers 1301a and 1301b, while the transmission path B includes two optical fibers 1302a and 1302b. The wavelength cross-connect device 1300 switches a signal in four directions (a total of four routes of #1 to #4) through four lines of the optical fiber 1301a to the optical fiber 1302b. More specifically, the signal can be switched between a route #1 and a route #2, between the route #1 and a route #3, between the route #1 and a route #4, between the route #2 and the route #3, between the route #2 and the route #4, and the between the route #3 and the route #4.
FIG. 60 is a schematic of a configuration of an optical cross-connect. The case of using an 80×80 matrix switch 1310, in which the number of inputs and the number of outputs of wavelengths are 80 (λ1 to λ80), is explained below as an example. If it is predicted that the number of final routes (the number of transmission paths) is four after introduction of the device, the number of fibers for a signal having one wavelength is eight lines as “4 lines (for transmission signals)+4 lines (when all the wavelengths are targeted for adding/dropping)=8 lines”. Therefore, the amount of 80/8=10 wavelengths is assigned to one matrix switch 1310.
If the number of routes upon initial introduction is two, input/output ports of the matrix switch 1310 for 40 lines obtained through “(2 lines (for transmission signals)+2 lines (for adding/dropping))×10 wavelengths” are used. Other input/output ports for the remaining 40 lines remain unused, which is wasteful. If prediction made upon the initial introduction is found incorrect and function expansion is required for the number of routes that is above the number predicted, the requirements may not be dealt with.